The laser interferometer is an important device for calibrating multi-axis machine tools, particularly including computer numerical control (CNC) and coordinate measuring machine (CMM) systems. In fact, the laser interferometer is currently the only device known to the inventor that is used for calibrating the latter types of equipment. With the rising demand for precise, machined parts in industry, the use of machine tooling in general, and CNC and CMM tools in particular, is increasing and calibrating these systems is becoming more and more important.
The traditional laser interferometer, however, is not a very user-friendly device. It can require almost eight hours to carry out a typical complete calibration of one CNC/CMM system today. This is because complete calibration involves six parameters for each axis, as well as three parameters for the interfacing planes, thus totaling at least 21 parameters that need to be measured, adjusted, re-measured, re-adjusted, etc., in a typical CNC/CMM system. In a traditional approach, a laser interferometer is used to measure these parameters, one-by-one, with optical and mechanical devices dedicated as each particular parameter is addressed in turn.
The users of CNC/CMM systems consequentially often have to bear the cost of a calibration specialist, as well as the loss of revenue, staff re-tasking, etc., while the CNC/CMM system is unavailable. Many users, therefore, choose to either extend the scheduled period between calibrations, or to only undertake calibration when it becomes obvious that there is no other alternative. The net result of all of this is often the production of poor quality parts and considerable waste.
To help the reader understand the 21 parameters that typically must be considered, and how a traditional laser interferometer is used to calibrate these in a CNC or CMM system, we now briefly discuss the principles involved. Additional detail can be found in user manuals for commonly used laser interferometer products, such as the 1100A from Excel Precision, Corporation of Santa Clara, Calif.; the 5529A from Agilent Technologies of Palo Alto, Calif.; or the MT10 from Renishaw, PLC, New Mills, Wotton-under-Edge, Gloucestershire, United Kingdom.
FIG. 1 (background art) is a conceptual block diagram depicting object movement. Basically, there are six degrees of freedom when an object is moving along a predetermined axis. These are: linear displacement along the axis, pitch, yaw, roll, horizontal straightness, and vertical straightness. Both CNC and CMM machines typically have three axes of movement. In a CNC system these are usually provided by two carriages or stages, one each for X- and Y-direction motion, and a spindle for Z-direction motion. In a typical CMM system these are usually provided by X, Y, and Z carriages. Both CNC and CMM machines thus have 18 parameters describing these. In addition, the perpendicularity between the three axes (X-Y, Y-Z, Z-X) also needs to be measured, bringing the total number of parameters that must be considered up to 21. Furthermore, current sophisticated machine tools may even have five axes of motion, for instance, permitting a spindle to be used “conventionally” in a Z-direction that is orthogonal to X and Y stages or to be rotated and be used at non-normal angles relative to the XY-plane.
In traditional laser interferometer based calibration schemes, displacement is measured using a “linear interferometer.” Pitch and yaw are separately measured using an “angular interferometer.” Horizontal straightness and vertical straightness are then measured separately using a “straightness interferometer.” The perpendicularity among the three axes is measured separately using an “optical square.” The measurement of roll is not achievable with a traditional laser interferometer.
The various parameters for calibration of multi-axis machine tools are therefore measured one-by-one, with different optics and mechanical mounting tools mounted and dismounted during the process. In particular, the laser head needs to be readjusted or repositioned every time that new optics are installed. And since positional accuracy is lost in each such change, a new reference has to be established whenever the laser head is moved.
It follows that shortening the time required for laser interferometer calibration of CNC/CMM systems is very desirable. One approach to this has been to use Excel Precision Corporation's dual laser beam 1100B laser calibration system. This system enables a user to more easily calibrate the six degrees of freedom for each moving axis of a CNC or CMM system, and it reduces the typical calibration time needed from eight hours to less than three. However, like traditional systems, the 1100B system can only measure one axis at a time, and the laser head used therefore needs to be moved and oriented and the alignment process started over for each axis.
Accordingly, it is desirable to have an approach, embodied in suitable methods and apparatuses, that will further reduce the calibration time for multi-axis machine tool systems by aligning all the moving equipment axes during initial set up, and that will permit measuring the parameters of interest without further adjustment of the laser head.